1. Field of the Invention
The present invention relates to a current source for a voltage regulator, and more particularly, to a current source capable of quickly adjusting an output current of a voltage regulator, and a voltage regulator thereof.
2. Description of the Prior Art
A voltage regulator utilizes a feedback circuit to maintain its desired output voltage. A voltage regulation capacitor is disposed in the output terminal of the voltage regulator to assist in regulation capability of the voltage regulator. The voltage regulation capacitor is responsible for converting pre-stored charge into a driving current that can be provided to the load driven by the voltage regulator when its current requirement changes rapidly. This can maintain stability in the output voltage of the output terminal. In order to allow the voltage regulator to support large current variations, a large voltage regulation capacitor should be applied, which increases the cost of the voltage regulator and also reduces the response speed.
The industry has therefore developed voltage regulators that do not require voltage regulation capacitors. These voltage regulators possess complex detection circuits that detect dynamic changes in the output voltage of the load terminal, and can dynamically adjust driving currents according to the detected output voltage variations. One common voltage regulator applies an N-type metal oxide semiconductor field-effect transistor (NMOS) as a power supply transistor instead of a P-type metal oxide semiconductor field-effect transistor (PMOS). The NMOS has a smaller tuning range in its output voltage than a PMOS; its gate-to-source voltage (Vgs) may therefore increase significantly when there is a rapid rise in the current requirement of the load terminal. This leads to a quick fall in the source voltage of the NMOS and failure to achieve a stable output voltage. U.S. Publication No. 2009/0212753 A1 and U.S. Pat. No. 7,106,033 B1 respectively teach another voltage regulator circuit structure, which utilizes a comparator to compare the output voltage with a reference voltage to enable an instant current source when the output voltage falls below a predefined level. The voltage regulator further applies another comparator to compare the output voltage with another reference voltage to disable the instant current source when the output voltage is sufficiently high or when an overvoltage due to an excessively large output current occurs. These types of voltage regulator require a more complex circuit design, which increases the cost and results in redundant power consumption. Furthermore, the instant current source is enabled after the output voltage has an evident fall, which limits the regulation effect in the output voltage. Since these circuit structures have two comparators controlled by two control loops, stability problems may easily occur.
As technology processes progress, the density of digital circuits becomes higher and their associated functionalities become more powerful, which results in larger instant currents. Modern voltage regulators without voltage regulation capacitors are not adequate for the required response speed. Even voltage regulators including voltage regulation capacitors are unable to provide a satisfactory voltage regulation effect due to parasitic resistances inside or outside the chip when the current requirement in the load terminal keeps increasing. Thus, there is a need for improvement over the prior art.